1. Field of the Invention
The present invention relates to gear pumps and hydraulic gear motors, in particular to a hydraulic system used to balance the axial thrusts in pumps and hydraulic motors with external gears of bi-directional type or multiple stages, wherein helical gears are provided.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Although specific reference is made to gear pumps hereinafter, the present invention also relates to hydraulic gear motors. Gear motors have the same construction as gear pumps, although they differ in the operating principle: whereas pumps are used to convert mechanical energy (torque applied to the drive shaft) into hydraulic energy (pressurized oil), motors are used to convert hydraulic energy (pressurized oil) into mechanical energy. The pressurized oil that is conveyed inside the hydraulic motor through one of the ports provided on the motor body acts on the toothed wheels by driving them into rotation; the torque is the output available at the shaft whereon a load is applied.
External gear pumps are commonly used in numerous industrial sectors, such as the automotive, earthworks, automation and control industries.
As shown in FIGS. 1 and 1A, a gear pump generally comprises two mutually engaged toothed wheels (1, 2). The toothed wheels (1, 2) are disposed inside a case (3) in such a way to define an inlet fluid area and an outlet fluid area.
One of the toothed wheels, which is defined as driving wheel (1), receives motion from a drive shaft, whereas the other toothed wheel, which is defined as driven wheel (2), receives motion from the driving wheel (1) it engages with. The toothed wheels (1, 2) are joined to respective shafts (10, 20) revolvingly supported by supports or bushes (4, 5).
In this description the term “front” refers to the side of the pump from which the shaft of the driving wheel protrudes, i.e. the inlet shaft that receives the rotation.
The pump comprises a front bush (4) that revolvingly supports a front portion of the shafts of the toothed wheels and a rear bush (5) that revolvingly supports a rear portion of the shafts of the toothed wheels. Each bush is provided with two circular housings that revolvingly support a portion of the shafts of the two toothed wheels.
A front flange (6) and a back lid (7) are fixed to the case (3) in such way to close the bushes (4, 5) and the toothed wheels (1, 2) inside a box composed of the case (3), the front flange (6) and the back lid (7). The front flange (6) is provided with an opening from which the shaft (10) of the driving wheel (1) comes out. Therefore a projecting portion (13) of the shaft of the driving wheel frontally protrudes from the front flange (6) in order to be connected to a drive shaft that transmits motion.
Gear pumps are volumetric machines because the volume comprised between the compartments of the teeth of the two toothed wheels and the external case is transferred from the inlet area to the outlet area by means of the rotation of the toothed wheels. Different types of fluid can be used, as well as different outlet and/or inlet pressure and pump displacement values.
The fluid used in the most typical application is oil, which is partially incompressible. Reference pressure values are typically the ambient pressure for the inlet pressure, whereas the outlet pressure reaches maximum values of 300 bar.
As shown in the example of FIGS. 1 and 1A, the toothed wheels (1, 2) have a straight external toothing, the same dimensions and a unitary transmission ratio.
Referring to FIG. 2, if toothed wheels with straight toothing are used, during operation the toothed wheels transmit a transmission force (F) that can be decomposed into a radial transmission force component (Fr) (shown in FIG. 2) directed in radial direction with respect to the axis of rotation of the toothed wheels and a transverse transmission force component (Ft) (not shown in FIG. 2) directed in radial direction with respect to the axis of rotation of the toothed wheels.
Referring to FIG. 2A, in these conditions, a pressure force (P) is generated in the inlet area (shown in bold in the left-hand side of FIG. 2A), which acts on the surfaces of the toothed wheels. The resultant of the pressure force (P) can be likewise decomposed in two components: a radial pressure force component (Pr) and a transverse pressure force component (Pt). In such a case, no force in axial direction is exerted on the toothed wheels.
The use of helical gears, when configured as disclosed in the international patent application PCT/EP2009/066127 or in the U.S. Pat. No. 2,159,744 or U.S. Pat. No. 3,164,099, allows for considerably reducing the noise and pulses induced by the pump in the hydraulic circuit.
It must be noted that in order to correctly engage two helical toothed wheels with the same geometrical features, the inclination of the helix must have a discordant direction.
FIGS. 3A, 3B, 3C and 3D disclose a gear pump with a driving wheel (1) and a driven wheel (2) with helical toothing. The use of toothed wheels with helical toothing generates axial loads or stress (Fa, Pa) during operation. The higher the helix angle ?b of the helical toothing is, the higher said axial loads or stress (Fa, Pa) will be (FIGS. 3A, 3B). The generation of the axial stress (Fa, Pa) is caused by the projection of the transmission forces (Fa) and the pressure forces (Pa) acting on the sections of the toothed wheels along the axial direction.
FIG. 3D shows the resultants (A, B) of all axial forces acting on the toothed wheels (1, 2), respectively.
If not opposed, the generation of the axial stress (A, B) considerably increases the specific pressure that is discharged on the bushes (4, 5), thus reducing both the mechanical efficiency because of losses by friction and the reliability and maximum pressure of the pump.
The problem of balancing the axial loads can be solved in different ways.
Referring to FIG. 4, the use of bi-helical gears is known to solve the problem of balancing the axial loads, because the axial forces (A, B) are directly balanced on the toothed wheels. Such a solution is impaired by several drawbacks: in fact, the higher constructional complexity of the bi-helical toothed wheels, together with the higher accuracy required during the construction of the high-pressure gear pumps or motors, makes such a solution cost ineffective.
An alternative method used to balance the axial forces is disclosed in the U.S. Pat. No. 3,658,452, wherein a right-hand pump (i.e. a pump with driving shaft with clockwise rotating right-hand helix) and driven shaft with left-hand helix are used.
Referring to FIG. 5 (which corresponds to FIG. 1 of U.S. Pat. No. 3,658,452) the axial forces (A, B) acting on the driving and driven toothed wheels (11, 12) of the pump are both directed towards the back lid (16) and opposed by hydraulic pistons (51, 52) disposed at the ends of the toothed wheels, which exert contrast forces (A′, B′). The hydraulic pistons (51, 52) are fed by means of passages (59, 60, 61) that connect the rear chambers (57 and 58) of the hydraulic pistons with the inlet area of the pump. The area of the hydraulic pistons (51, 52) must be suitably dimensioned in order to balance the axial forces (A, B).
The axial forces (A, B) acting on the toothed wheels are generated by the contribution of two factors: the axial component of the pressure (Pa) (FIG. 3B) and the axial component of the force (Fa) generated by the torque transmission from the driving wheel to the driven wheel (FIG. 3A). Regardless of the direction of rotation and the direction of the helix used for the wheels, the forces (Pa and Fa) are always concordant on the driving wheel, whereas the forces (Pa and Fa) are always discordant on the driven wheel.A=Pa+Fa[N]  (1)B=Pa−Fa[N]  (2)
If a pump with helical gears according to the prior art in right-hand rotation (clockwise-rotating driving shaft) is considered and a driving shaft with right-hand helix is used (FIG. 5), at a known running speed, the absorbed torque at the driving shaft is:
                    Mt        =                                            V              ·              P                                      20              ·              π              ·                              η                m                                              ⁡                      [            Nm            ]                                              (        3        )            
V=Displacement [cm3/rev]
P=Pressure difference between inlet and outlet [bar]
nm=Hydro-mechanical output (experimentally obtainable value)
Assuming that half of the torque is transferred to the fluid by the driving wheel during its pumping action, the torque transmitted to the driven wheel Mtcto is half of the total torque.
                              Mt          CTO                =                              Mt            2                    ⁡                      [            Nm            ]                                              (        4        )            
The axial transmission force Fa generated by the helical toothed wheels is:
                    Fa        =                                                            1000                ·                                  Mt                  CTO                                                            Dp                2                                      ·                          Tan              ⁡                              (                β                )                                              =                                                    50                ·                V                ·                P                                            π                ·                Dp                ·                                  η                  m                                                      ·                                          Tan                ⁡                                  (                  β                  )                                            ⁡                              [                N                ]                                                                        (        5        )            
Dp=Operating pitch diameter of toothed wheels [mm]
β=Inclination angle of helix [°]
Because of the known action and reaction principle, the force Fa acts on the driving and driven wheel with the same intensity, but with opposite direction.
The axial force generated by the pressure Pa is the resultant of the pressure along the axial direction:
                    Pa        =                                            h              ·              l              ·              P              ·                              Tan                ⁡                                  (                  β                  )                                                      10                    ⁡                      [            N            ]                                              (        6        )            
h=Tooth height [mm]
l=Ring width [mm]
In view of the above, the force Pa has the same intensity and the same direction on both toothed wheels. According to the most typical dimensioning of the toothed wheels, Pa>Fa and consequently the forces F1 and F2 always have a concordant direction.
The diameters ΦA and ΦB of the compensating pistons are obtained from the formulas (7) and (8):
                              Φ          A                =                  2          ·                                                                      10                  ·                  A                                                  π                  ·                  P                                                      ⁡                          [              mm              ]                                                          (        7        )                                          Φ          B                =                  2          ·                                                                      10                  ·                  B                                                  π                  ·                  P                                                      ⁡                          [              mm              ]                                                          (        8        )            
Both forces Fa and Pa linearly depend on the value of the inlet pressure P (see formulas (5) (6)). Consequently, after calculating the diameter of the compensating pistons, the axial forces are completely balanced at any value of the pressure P.
The use of the compensating pistons is a rather inexpensive and easy-to-make solution because the work operations and the parts are simple and reliable. The precepts disclosed by the U.S. Pat. No. 3,658,452 can solve the problem of balancing the axial forces only in case of monodirectional motors, in which the resultant forces A and B must be always directed towards the back lid (see FIG. 5), (i.e. in case of a right-hand pump with right-hand driving gear and left-hand driven gear, or in case of a left-hand pump with left-hand driving gear and right-hand driven gear).
However, some hydraulically controlled applications require the use of bi-directional or multiple stage hydraulic pumps or gears.
The use of bidirectional pumps (with two flow directions) allows for inverting the rotation of the driving shaft, thus inverting the direction of the oil flow and the high and low pressure areas, inverting, for instance, the motion of hydraulic actuators. Likewise, the use of bidirectional motors is useful in the applications that require inverting the direction of the torque available at the outlet shaft of the hydraulic motor.
FIG. 6A shows the distribution of the axial forces in case of a bidirectional pump, in an operating condition in which the axial forces A and B are directed towards the front flange. In such a case, the solution disclosed in U.S. Pat. No. 3,658,452 is not applicable because the inversion of the motion and of the inlet side with the outlet side results in the inversion of the axial forces (A, B) acting on the toothed wheels (1, 2), as shown in FIG. 6B. In such a case the axial forces (A, B) are directed towards the front flange (6) and not towards the back lid (7). Because of the inevitable projecting portion (13) of the shaft of the driving wheel (1) that protrudes from the front flange (6), the axial force (A) on the driving wheel (1) can no longer be balanced by means of a hydraulic piston, like in the solution shown in FIG. 5.
The same situation is found in a hydraulic motor with a high-pressure fluid inlet side and a low-pressure fluid outlet side. In such a case, there are no driving wheel and driven wheel, but simply a first toothed wheel (1) and a second toothed wheel (2). Moreover, the projecting portion of the shaft (13) is adapted to be connected to a load, not to a motor.
FIG. 7 shows a multiple two-stage pump comprising a front stage (SA) and a rear stage (SB). For the sake of clarity, FIG. 7 shows a two-stage pump, but the solution can be applied also to a higher number of stages. A multiple pump is necessary to connect multiple independent circuits to a single power take-off. In such a case the pumps are connected in parallel and the rear stage (SB) receives the necessary torque by means of a mechanical connection (500) (such as Oldham coupling or splined coupling), from the shaft of the driving wheel of the front stage (SA). Also in the case of multiple pumps, the solution disclosed in the U.S. Pat. No. 3,658,452 is not applicable because an end portion (T) of the shaft of one of the toothed wheels of the front stage (SA) is engaged to transmit the motion to the rear stage (SB). In fact, the front stage (SA) cannot be provided with a closed back lid because the end portion (T) of the shaft of a toothed wheel must protrude in the back to transmit the motion to the rear stage (SB).
In general, the precepts disclosed by the U.S. Pat. No. 3,658,452 are not applicable when the axial forces (A, B) are directed towards a side of the pump that is crossed by the shaft of a toothed wheel.
The purpose of the present invention is to remedy the drawbacks of the prior art, by providing a hydraulic system to balance the axial forces in gear pumps or hydraulic motors with helical toothing of bidirectional or multiple stage type.